Three (3) kinds of alpha particle detectors for alpha particle counting are available in the prior art: the scintillation counter, the gas counter including both Geiger and proportional types, and the solid state junction counter.
The scintillation counter was historically the first to be utilized in early experiments on radioactivity in which the scintillator was viewed with a microscope to count the individual flashes of light produced by each alpha particle stopping event. In the modern scintillation counter, scintillation material is deposited on the photocathode of a photomultiplier tube which amplifies the signal and provides information about the energy of the alpha particle in addition to simply counting the alpha particle. The acceptance window must be thin enough to pass low energy alpha particles but thick enough to form an opaque light seal. The result is a window which is vulnerable to scratching, scores and puncture. Clearly, the scintillation counter is not well suited for alpha particle detection and counting under severe conditions, such as those present in military operations in the field.
Gas-filled alpha particle detectors use a specific gas as the detector material depending upon whether the mode of operation is as a Geiger counter or as an ionization/proportional counter. In either case, the working gas is hermetically sealed. Entrance to the ionization zone by the incoming alpha particle is through a thin, fragile plastic or metal window. The output signal pulse is constant in the Geiger counter operation but is related to the energy of the alpha particle in the ionization or proportional counter operation. The existence of delicate windows for the entrance apertures for the incoming alpha particles make gas filled counters unsatisfactory for use in a rough and hazardous environment, such as that of military operations in the field.
The junction counter is a solid state p-n junction with a reverse bias which collects ionization charge from the passage of an alpha particle through the depletion layer. It can be made compact and portable. The limitation of the junction counter lies in the stringent requirements for avoiding scratching and abrasion of the coating over the metallic electrode window surface of the detector. This electrode is light sensitive and the coating serves to block ambient light. The coating is easily broken resulting in a light leak. Equally important, the active surface of the detector must be kept free of moisture and dust. At least one amplification stage is required to register an event whereas several stages of amplification are typically necessary to extract energy information. Because of the vulnerability of the active surface and the care which must be given to it, the junction detector is not suitable for routine service under extreme conditions such as those found in military field operations.
None of the three (3) types of alpha particle counters currently available is a satisfactory choice for alpha particle detection and counting under severe or hazardous conditions. This shortcoming has been recognized by the military services in the form of a Joint-Service Operational Requirement (JSOR) entitled "Alpha Radiation Detection and Measurement System" issued Feb. 12, 1987. The purpose of the required system is stated:
"The desired alpha radiation detection and measurement system will be used worldwide for peacetime operational health physics applications, i.e. nuclear weapons accidents contamination control, and `hot line` monitoring."
The JSOR defines the need for an alpha particle detector as follows:
"The military services and defense agencies require a rugged state-of-art alpha radiation detection system to detect and quantify alpha radiation levels, e.g. plutonium, uranium and daughter products, with sufficient sensitivity at low levels for the release for uncontrolled use and at high levels to meet operational safety and health requirements."
"Alpha emitting radioisotopes possessed by the military services and defense agencies can be involved in an accident during storage, maintenance, transport, or other operational activities. These alpha emitters could be dispersed in the environment producing radioactive contamination and pose a potential health hazard."
The JSOR concludes that existing alpha particle detection and counting systems are not suitable for the needs of the military.